According to a primary aspect, the invention more particularly relates to an installation provided with a free suction fan, the inlet(s) of the fan being at least partially defined peripherally by an annular inlet wall portion that tapers in the inlet direction, and the flow rate being-measured on the suction side of the fan in the vicinity of the inlet opening(s) thereof. Such a method, as well as an apparatus for carrying out the method is known from EP-A1-0419798 (Gebhardt Ventilatoren GmbH).
The performance of ventilation installations is dependent to an essential degree on the total flow rate. A given minimum flow rate is thus required in any ventilation installation for achieving the desired indoor climate, particularly with respect to low pollution percentages in the room air and desired room temperature by regulated supply of heat or cooling with the air.
The greater requirements made on the indoor climate, the more important it is to be able to measure, monitor and regulate the total and partial flows in the ventilation installation. If the total flow rate from a central unit decreases by 10%, the partial flows to each room will also decrease by 10%. If monitoring of the total flow rate is enabled, the partial flows in the entire ventilation installation can thus be indirectly monitored as well.
Several methods are known for flow measurement, particularly partial flows, but these methods either require an extra pressure drop with accompanying energy increase, generation of noise and increased operational costs, or they require high flow rates for achieving sufficient measurement accuracy. Such high flow rates are not normally present in ventilation ducts, and in addition there are often obstacles, e.g. in the form of bends close to the measuring point. Therefore, it is generally not possible to achieve sufficient measurement accuracy with certain simple flow meters, such as so-called Prandtl tubes (which measure dynamic pressure, i.e. the difference between total pressure and static pressure) or temperature-responsive electrical components (e.g. a resistor, the resistance of which depends on the temperature and thus also on the flow rate of the cooling air). For satisfactory measurement accuracy within a large flow range there is further required that the flow meter is placed in a straight duct section with a distance of about 5 duct diameters before and about 3 duct diameters after the measurement point.
In accordance with prior art, and as indicated above, the flow rate in a ventilation installation may be determined on the basis of pressure-drop measurements which can be made at different places in the ventilation installation, e.g. in a duct system connected to a central unit, as is proposed in the published Swedish patent application SE-A-8704163-8 (AB Bahco Ventilation). A pressure difference across a component in the duct system is measured here with the aid of pressure measurement outlets, the pressure drop across the component then being proportional to the square of the flow. The pressure measurement outlets are connected via hoses to a pressure-sensing means in a meter with a pointer for visual indication of the flow. This measurement method is however burdened with the disadvantage that comparatively poor measurement accuracy is obtained, partly due to the comperatively low flow rate in the ducts and partly due to practical installation difficulties.
Another method of flow determination is described in the Swedish patent specification SE-C-455 442 (AB Bahco Ventilation). In this case a filter in a central unit is exchanged for two perforated plates serving as constriction means, pressure sensors then being used to measure the different pressure drops which occur with the filter in place on the one hand, and the constriction plates on the other. On the basis of the flows which have already been measured with the constriction plates located in a similar unit, the flow rate is interpolated or extrapolated when the filter is in place, e.g. graphically with the aid of a diagram.
This method also gives comparatively poor measurement accuracy, and it cannot be used for continuous measurement during operation of the installation, at least not without considerable complications and work from personel.
Yet another known method is described in the published Swedish patent application SE-A-8701663-0 (Flakt AB), the pressure drop measurement being carried out on the suction side of a suction fan in a ventilation installation.
The fan is placed in an apparatus housing and on its pressure side is connected to a duct system. A constriction means is arranged in the inlet portion of the housing on the suction side of the fan and has two pressure tappings connected to a differential pressure measurement device for determining the pressure drop and the flow rate.
The constriction means, e.g. in the form of adjustable baffles is adjustable between a completely open position and a maximum constriction position, i.e. the measurement position, which enables determination of an empirical graph of the relationship between the measured pressure difference in the measurement position and the corresponding flow rate.
This known apparatus thus requires a considerable constriction of the total flow rate during measurement while the apparatus is in operation, which results in increased need of energy, increased operational costs and disturbing noise.
Both in measurement of the total flow rate in or in connection with an air-conditioning unit (apparatus housing) and in measurement of the partial flows in ducts it has therefore been necessary to compromise energy demand and measurement accuracy. No suitable method for readily determining the total flow rate in a ventilation installation has been provided so far. An attempt has been made to measure the pressure drop in a measurement chamber provided with constrictions and situated in the outlet part of a fan (PCT/FI88/00149, publication No. W89/02581-Imatera) but even this method has turned out to be unsuitable, due to complicated apparatus and comparatively high costs.
Attempts have also been made to measure the local flow rate in the vicinity of the inlet opening of a fan. See EP-A1-0419798 (Gebhardt Ventilation GmbH, mentioned in the opening paragraph) and JP-A-59 13 1116 (Nihon Furooseru K.K.) In both these cases, the measurement is carried out by means of a tapping hole, made in the annular inlet wall portion, and an adjoining pipe or hose for sensing the static pressure at the surface of the inlet wall portion of the fan. However, in the region closest to the surface of the inlet wall portion, the air flow is somewhat irregular and possibly subject to frictional disturbances, which depend on the exact geometrical configuration and the smoothness of the surface and the flow rate. Therefore, the static pressure measured in such a way is generally not representative of the total flow rate, in particular when using damper control of the flow at a constant rotational speed of the fan. Moreover, of course, such a method can only be used upon modifying the inside structure of ordinary fans. There is also risk of clogging of the tapping holes being freely exposed in the inlet.